Molten salt bath apparatus for electrolytic cleaning of metals



y 1960 R. H. SHOEMAKER ETAL MOLTEN SALT BATH APPARATUS FOR ELECTROLYTICOriginal Filed March 7, 1955 CLEANING OF METALS 3 Sheets-Sheet 1 f I a II T\ 2 I I I I 2 I r INVENT0R5. W N. M, By Ma./m,

\ Mg- MGM y 1960 R. H. SHOEMAKER EIAL 2,936,278

MOLTEN SALT BATH APPARATUS FOR ELECTROLYTIC CLEANING OF METALS OriginalFiled March '7, 1955 3 Sheets-Sheet 2 O 5 f9. Q 2 =5 38 q SE15 &

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May 10, 1960 R. H. SHOEMAKER ET L MOLTEN SALT BATH APPARATUS FORELECTROLYTIC CLEANING 0F METALS Original Filed March 7, 1955 3Sheets-Sheet 3- IN V EN TORS WA By a air/v N MOLTEN SALT BATH APPARATUSFOR ELEC- TROLYTIC CLEANING OF METALS Original application March 7,1955, Serial No. 492,358. Divided and this application July 1, 1957,Serial No. 669,281

1 Claim. (Cl. 204-206) This application is a division of co-pendingapplication Serial No. 492,358, filed March 7, 1955 and now abandoned,and relates to the cleaning of metals by electrolytic molten salt bathsand aims to provide novel apparatus for this general purpose.

One object of the present invention is to provide a molten salt bathapparatus comprising essentially two separate baths, one of which isconnected as an anode and the other as a cathode, with the moving metalstrip being cleaned, immersed in both baths, serving as a means forelectrically connecting the two baths and completing the circuit betweenthe anode bath and the cathode bath.

A still further object is to provide a molten salt bath of 1 atent thisgeneral character wherein anodes and cathodes are 1 spacedlongitudinally and alternately along the moving strip in the bath. Theseelectrodes, alternate groups of which are connected to opposite sides ofthe circuit to function as anodes and cathodes respectively, are in theform of transversely arranged metal rods, and may be positioned ingroups above and below the moving strip.

Because it is desired to have the moving strip as close as possible tothese electrodes to improve efliciency, it is a further object of thisinvention to provide guide and guard rolls of insulation material withwhich the strip may come in contact and which guard the strip againstcontact with the electrodes and yet guide the strip very close to butslightly spaced from these electrodes.

Accordingly, it is an aim of this invention to provide guard means inthe form of an insulation member which insulates the work piece frompart of the apparatus with which the workpiece should not come incontact. In the embodiment described above, the insulation member guardsthe work piece from coming in contact with the electrodes.

Notwithstanding the arrangements outlined in the preceding paragraphs afurther definite object exists whereby the operation could besuccessfully executed by the mere advantageous positioning of electrodeand strip by the complete omission of insulation, if so desired.

A still further object is to provide an apparatus comprising twoinsulated baths, each having an immersed electrode of opposite polarityto the other, with the metal strip completing the circuit between theopposite polarity electrodes.

A still further object is to provide various novel arrangements forconnecting the strip and the bath in electrolytic continuous stripcleaning.

Further aims and objects of the invention will be under stood uponreference to the appended drawings, which show various embodiments.

In these drawings:

Fig. 1 is a side elevation view of a metal cleaning apparatus employingtwo baths connected to opposite sides of the circuitto function as anodeand cathode respectively, and with the metal strip being the only meansconnecting the two baths electrically.

Fig. 2 is a top plan view of Fig. 1.

Fig. 3 is a small scale view of a modification where two baths of Fig. 1are incorporated in a single metal tank.

Fig. 4 is a side elevation view to small scale of an electrolytic moltensalt cleaning bath employing electrodes disposed along the moving stripabove and below it and connected alternately in groups to opposite sidesof the circuit, with guard and guide rolls of insulation materialincluded in the combination.

Fig. 5 shows an arrangement directly to the strip.

Referring now to the drawings, it will be observed that Figs. 1 and 2show a molten salt cleaning apparatus of the character developed forcontinuously cleaning of strip metal such as steel by the electrolyticprocess described in the H. G. Webster Patent No. 2,468,006, dated April19, 1949.

According to that patent, the metal cleaning process contemplatessubjecting the work-piece, while immersed in a molten salt bath,alternately to opposite sides of a direct current circuit whereby theworkpiece functions alternately as anode and cathode for electrolyticcleaning.

Figs. 1 and 2 show the work piece in the form of a moving strip ill ofsteel or the like being cleaned while continuously moving through moltensalt baths 11 and 12 arranged longitudinally in the line of travel ofthe strip. These baths are electrically separated and the metal tanksforming these baths are connected as shown diagrammatically in Fig. 2 toopposite sides of a direct current circuit so that the tanks 11 and 12each function respectively as a cathode and an anode, and consequently,the metal strip 10 passing through each bath is first subjected tocathodic conditions'and then anodic, and also serves to interconnect thebaths electrically.

Transversely disposed and mounted in the tank 12 are rollers 14 and 14afor supporting and guiding the strip immersed in its movement throughthe baths. These rollers are normally formed of metal and consequentlywould form electrodes for establishing electrical contact through themolten salt bath between the moving strip 10 and the tanks 11 and 12. Insuch a case electrical current to tank 12 would, to a large extent, beshunted across the rollers and thereby bypass the salt bath. Therefore,it is desirable to insulate the rollers from the tank to obtain maximumcurrent flow through the salt bath. As seen in the drawing the rollersare mounted on bushings 15 and 15a which are of non-conducting material.

In tank 11, a support and guide roller 16, mounted in bushnig 17, isprovided. Since the cleansing action of the process takes place in tank12, it is not important whether or not this roller 16 is electricallyinsulated from the tank 11. The operation will not be materiallyafiected in either case.

The circuit between bath 11 and bath through the metal strip 10 beingcleaned. The arrangement described in the foregoing provides a simpleyet efiicient apparatus for electrolytic cleaning of continuously movingmetal strip while immersed in a molten salt bath.

Fig. 3 shows an apparatus comprising two insulation tanks 21 and 22which provide two baths through which the strip moves. These tanks 21and 22 are disposed within a single metal tank 23.

The rollers indicated generically at 2 3 for supporting the strip andguiding the strip while immersed in the baths are not connectedelectrically to the metal tank 23 and hence have no useful functionelectrically. Instead, independent electrodes such as are shown at 25and 25 are employed for the two baths 21 and 22. Each electrodecomprises a group of metal rods supported and immersed within the bathadjacent to but out of contact with the strip and the electrodes 25 and26 are electrically connected to oppowherein current is applied 12 iscompleted \2 site sides of the electric circuit. The strip completes thecircuit.

This apparatus, described above may be further simplified by omittingthe electrodes in one bath, such as electrodes 25 in tank 23 and simplyconnecting the electrical circuit directly to tank 23 without insulationin the same manner as the connection to tank 11 in Fig. 1 is made.

Fig. 4 shows an arrangement including a metal tank 31 with supportingand guiding rollers 32 along which the immersed strip 33 may passthrough the bath while immersed in the molten salt bath.

Longitudinally spaced along the strip, above and below it, are groups oftransverse metal rods forming electrodes 34 and 35 similar inconstruction and function to the electrodes 25 and 26 described inreference to Fig. 3. However, alternate groups of electrodes 34 and 35of Fig. 4 are connected to opposite sides of the circuit. In theembodiment shown, four groups of rods are used to provide fourelectrodes of which the first and third are connected as cathodes andthe second and fourth as anodes.

Because it is desired to maintain the distance between the electrodesand the strip as small as possible, the rods on opposite sides of thestrip are spaced as close to each other as possible without howevercoming in contact with the strips. Because the strip is flexible and maymove and twist in its passage through the bath accidental contact maysometimes occur with consequent short circuiting, arcing, and burningofparts. To avoid this there are included guide and guard rollers 38 onopposite sides of the strip which guard the strip in its motion throughthe electrodes and the bath but which prevent the strip from coming intocontact with the electrode. These guard rollers may be in the form oftransversely arranged metal rods supported at their ends on the tankwith each rod being surrounded for its full length or at intervals byperforated disks or rings or the like of insulation material which formin effect insulation rollers or guides along which the strip may pass.

If desired, shields, either metallic or insulating, might also beemployed.

Fig. shows an arrangement wherein tank 41 contains electrodes 42 onopposite sides of the strip 43 passing beneath rollers 44 through thebath 45 with electrodes 42 insulated from the tank 41, from the strip43, and from rollers 44, and connected to one side of a D.C. source. Theother side of the DC. source is connected to rollers 47 outside'the tankover which passes the strip '43 whereby the latter is connected to suchlatter side of the DC. source, with the contact between strip 43 androllers 47 being of suflicient area and quality to insure against arcingbetween such strip 43 and rollers 47 and consequent burning.

As further described in the Webster Patent 2,468,006, work pieces,deposited in a perforated metal basket, may be immersed in the bath. Anelectric current is then applied to that metal basket of such a naturethat the work piece, or the basket, forms a cathode, with the bathreceptacle or any other element forming the anode.

Thereupon the current is reversed so that the work pieces or the basketcontaining them is made the anode, and the receptacle or some otherelement is made the cathode.

Thereafter the current is reversed again so that the work pieces or thebasket containing them is a cathode. This is the final step in thepreparation of the work piece surface for the further processing.

The bath is of a molten salt type and contains a composition comprisinga mixture formed by fusing or melting together a predominant or majorproportion of a caustic alkali such as sodium hydroxide or potassiumhydroxide in the proportion of 75% to 90% and minor proportions of 7% to13% sodium chloride, sodium fluoride used in proportions 2% to 6% andsodium aluminate used in proportions of 0.5% to 5%, all proportions byweight.

The preferred bath is a molten salt bath consisting of about 85% ofsodium hydroxide, about 10% sodium chloride, about 4% sodium fluoride,and about 1% sodium aluminate, by weight.

The sodium fluoride is used for purposes of removing sand inclusionsfrom the metal surface. Where the metal has no sand as impurity thesodium'fiuoride may be omitted. For sand-free metal the bath may have apreferred composition 89% caustic soda, 10% sodium chloride and 1%sodium aluminate.

As variations in the process, it is possible to repeat the steps of theprocess, as much as needed, without taking the work piece out of thebath, and merely by continued applications of current, reversing thecurrent from time to time. This may be prolonged to any extent needed inorder to continue the treatment of the metal and to attain a higher andhigher degree of purity of the ferrite surface until the desired maximumis attained.

For other types of work pieces, other variations are possible. If thework piece is covered with scale, partly or wholly, the current is firstapplied with the work piece as a cathode; however, if there is no scale,this step may be omitted and the first application of current'may bewith the work piece as the anode.

If the casting has only sand on its surface, the same may be removed byusing the work piece as the cathode.

If the work piece contains nickel or other ingredients, which areinsoluble in the bath, even with the electric current applied, then wefirst use the process hereof and thereafter immerse the work piece in asuitable acid to remove the nickel or similar ingredients and then werepeat the process hereof to the extent necessary to get the desiredresults.

The temperature of the bath may be between 600' and 1000 degreesFahrenheit, but optimum results have been obtained at 850-900 degreesFahrenheit.

The electrical current is slow alternating or successively reverseddirect, of 4 to 6 volts, with a preferred intensity of 50 amperes persquare foot. We have found that five minutes has proven satisfactorywhen the work piece is an anode and ten to thirty minutes when the workpiece is a cathode. 7

Now having described the apparatus herein disclosed for the cleaningof'metal electrolytically in molten salt baths, reference should be hadto the claim which follows.

We claim: I

A molten salt bath apparatus for cleaning of metal strip by passing thesame continuously through molten salt contained in the apparatusas anelectrolysis bath under electrolyzing conditions for removal of oxidesand scale thereon, comprising an elongated metal tank of substantialdepth adapted to contain molten salt therein as an clectrolyzing bath,metallic guide rollers near the inlet and outlet ends of said tanksupported from the walls thereof, but electrically insulated therefrom,said rol ers being adapted to guide metal strip into and out of saidtank, constraining said strip to a fixed path of movement through themolten salt therein, several electrodes, each of opposite polarity, in alongitudinally dis posed series separated one from the next, eachelectrode being supported at an intermediate height of said tank forimmersion in the molten salt out of direct electrical contact with thewalls thereof, each of the series of electrodes being supported closelyadjacent to the path or" travel but out of direct contact with the metalstrip passing through the bath continuously in a fixed path,

each electrode comprising a group of several cylindrical,

rods mounted in a pair of parallel planes transverse to the path oftravel of said metal strip, each rod being mounted in a plane closelyadjacent to another rod of the group and electrically connected to eachother red at its end whereby to form a unitary electrode of two iancs ofrods between which the metal strip may pass and he subjected on bothsurfaces to large current densities from the planes of rods, a series ofguide rollers dis posed in tangential pairs, each pair of rollers beingmounted between and adjacent to groups of electrodes of oppositepolarity and there positioned to support and guide said metal strippassing through the nips of each pair of said tangential rollers, saidtangential rollers comprising, at least on their arcuate surfaces,electrically non-conductive material chemically resistant to attack bysaid molten salt at the temperature of said bath, whereby saidtangential rollers guide and rigidly support said metal strip in closepassage to said electrodes for close electrical influence at highcurrent densities, but out of actual electro-conductive contacttherewith.

References Cited in the file of this patent UNITED STATES PATENTSKirschner Dec. 2, Kelvie Apr. 14, Fosburg Sept. 8, Yerger et a1. July11, Hall Feb. 3, Forsberg June 19, Webster Apr. 19,

